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Synchronous speed

In an induction motor the stator windings create a rotating magnetic field. How fast it rotates depends on the supply frequency and the number of poles. More poles give a lower speed.

ns=120 fpn_s = \frac{120\,f}{p}synchronous speed

Symbols

nsn_ssynchronous speedrpm
ffsupply frequencyHz
ppnumber of poles

Example

4-pole motor at 50 Hz:

ns=120⋅50/4=1500n_s = 120\cdot 50/4 = 1500 rpm.

The number of poles is always even. One pole pair is one north and one south pole.
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Part of Electric Power and Machines: Electric motors.